1. Field of the Invention
The present invention relates to the field of energy harvesting. More particularly, embodiments of the present invention relate to devices comprising at least two modes of energy harvesting, such as inductive and magnetostrictive mechanisms. Specific embodiments of the invention provide devices useful in structural health monitoring systems and other sensor technologies typically deployed on structures with limited accessibility and allow for wireless and/or remote monitoring.
2. Description of Related Art
Condition based health monitoring systems find application on a wide spectrum of platforms including railways, trucks, bridges, and ships. The three stages of condition-based maintenance (CBM) are diagnostics, prognostics, and maintenance scheduling. The diagnostics involves real time fault monitoring and diagnosis, background studies, and fault analysis.
One systems approach to the design of CBM includes failure identification and its criticality, failure analysis and decision making, failure classification, prediction of failure evolution, scheduling of required maintenance, and collaboration with the logistics. See G. Vachtsevanos, F. L. Lewis, M. Roemer, A. Hess, and B. Wu, “Intelligent Fault Diagnosis and Prognosis for Engineering Systems” (2006); see also G. Vachtsevanos, F. Rufus, J. V. R. Prasad, I. Yavrucuk, D. Schrage, B. Heck, and L. Wills, Software Enabled Control: Information Technologies for Dynamical Systems, pp. 225-252 (2005).
The pre-stage of diagnostics involves machine sensors, data collection and data transfer for further processing. Many of these components used in diagnostics are currently battery powered which increases the operation cost and adds additional complexity. With increasing demand for wireless sensor nodes in automobile, aircraft and rail applications, the need for energy harvesters has been growing. In these applications, energy harvesters provide a more robust and inexpensive power solution than batteries. Thus, energy harvesting has emerged as an effective way to either reduce the number of batteries or increase their lifetime. As is the case with any battery-powered device, existing battery powered sensor systems typically have a battery housing for accommodating a limited number and type of batteries. Due to size constraints, sensor systems disposed in remote locations and/or on remote structures or vehicles are typically installed on surfaces where it is not possible to accommodate an additional energy harvester module. Accordingly, it is desired to provide multimodal vibration energy harvesters that have a similar form factor as that of a battery which makes integration easier with existing electronic components. In this manner, the energy harvester may be integrated into the existing battery housing thereby replacing one or more batteries, or an additional energy harvester structure about the same size as the existing battery housing may be mounted in the vicinity without being too bulky.
The dominant vibration magnitudes available within railways, trucks, bridges, and ships typically exist at frequencies below 20 Hz. Within this frequency range the vibration frequency can fluctuate requiring the capability to harvest at a broadband of frequencies. Thus, it would be desired to develop a low frequency and broadband vibration energy harvester.
In vibration energy harvesting, there are predominately two types of harvesters, namely, cantilever beam and magnetic levitation based designs. Cantilever beam based harvesters typically optimally operate at frequencies greater than 50 Hz. The difference in frequency range between the two harvester types is due to stiffness magnitude control. The stiffness created by repulsive magnets in a magnetic levitation system can be decreased by decreasing the strength of the outer magnets or distance between top and bottom magnets, whereas cantilever beam stiffness is determined by the beam geometry. Cantilever beam stiffness cannot be decreased to the lower levels achieved by magnetic levitation harvesters without sacrificing the structural integrity of the beam or by increasing the cantilever length increasing the size of the harvester to impractical levels. Another advantage inherent to the magnetic levitation systems is a non-linear stiffness profile. This is due to the repulsive force between magnetic poles increasing by the square of the distance between them. The non-linear stiffness profile causes magnetic levitation harvesters to have a non-linear frequency response which allows for harvesting more power within a broad range of frequencies as compared to linear frequency response.
In order to enhance the power density of existing energy harvesters, a variety of multimodal energy harvesting techniques have been proposed. Generally, multi-modal energy harvesters can be categorized as: (i) Multi-Source Energy Harvester (MSEH), (ii) Multi-Mechanism Energy Harvester (MMEH), and (iii) Single Source Multi-Mode Energy Harvester (S2M2EH). Particularly desired are multimodal energy harvesting devices with magnetic levitation and magnetostrictive capabilities.